1. Field of the Invention
The present invention relates to a method of forming a polycrystalline semiconductor film, and in particular, to a method of forming a polycrystalline semiconductor thin film serving as an active layer for a thin-film transistor.
2. Description of the Related Art
As a material for fabricating semiconductor devices which exhibit performance comparable to that of fabricated with single-crystal silicon (generally single-crystal semiconductors) on insulating substrates such as glass, polycrystalline silicon thin films (generally polycrystalline semiconductor thin films) formed by laser annealing technique have been used. To form such polycrystalline silicon thin films, a method has been invented which irradiates an amorphous silicon thin film (generally amorphous semiconductor thin film) with laser light exhibiting a predetermined light intensity distribution. With this method, a poly Si thin film with its grains continuously grown in a predetermined direction can be obtained by melting the amorphous Si once and afterward by controlling the position of the generation of initial nuclei and the timing of the recrystallization.
Various methods of controlling the position and timing of the crystal regrowth have been proposed. One of these methods is a method of recrystallizing a silicon thin film on the basis of phase-modulated excimer laser annealing (PMELA). With PMELA, a phase shift mask (generally a light modulating element) is used to modulate the distribution of the intensity of the incident laser light projected to the amorphous silicon thin film. Specifically, the said distribution of the intensity of the incident laser light is such that the light intensity (irradiation energy) has the minimum value at a desired position on the amorphous silicon thin film and varies monotonously between the position of the minimum value and the position of a maximum value.
Thus, a temperature distribution is generated in the amorphous silicon thin film; the temperature distribution is such that temperature varies monotonously between the position of the minimum light intensity value and the position of the maximum light intensity value. Owing to this temperature distribution, after the amorphous silicon is melted, crystal growth occurs continuously in a direction from where the temperature is the lowest to where the temperature is the highest, a device-grade polycrystalline thin film with continuously grown, large grain polycrystalline silicon, that has the crystallinity comparable to single crystal silicon can be obtained.
As an example of the conventional technique, Jpn. Pat. Appln. KOKAI Publication No. 2000-306859 discloses a method of recrystallization by irradiating a semiconductor film with laser exhibiting intensity distribution with an inverse peak pattern generated via a phase shift mask. With this conventional technique, the grain growth starts from two groups of initial nuclei that each are respectively arranged on one of two parallel lines. The grain growth completes when grains collide against one another at the center of the said two parallel lines. At the point where grains collide against one another, the thickness of the silicon film is increased compared to that of the other areas, forming a linearly continuous hillock, as the result. When such a recrystallized silicon thin film is used to form an active layer for a thin-film transistor (TFT) on an insulating substrate, the said hillock portion of the silicon may cause two adjacent TFTs to be short-circuited.